A conventional press machine, specifically, a mechanical press, has been disclosed that includes a connection swingably connected to each of two eccentric portions of a crank shaft borne rotatably on a frame, and a lever connected to a slide, in which the connection and the lever are movably connected to the crankshaft. See Japanese Patent Appln. Public Disclosure No. 8-118082; Japanese Patent Appln. Public Disclosure No. 2000-202691; and Japanese Patent Appln. Public Disclosure No. 2002-35993.
According to the above documents, a load providing a vertical force is converted into equal loads providing horizontal forces in directions opposite from each other relative to a crankshaft, by symmetric linkages to eccentric portions of the crankshaft. This configuration results in the horizontal loads of the crankshaft being mutually cancelled to maintain a balance in the loads, thereby enabling efficient transmission of power from a power source to facilitate rotation of the crankshaft. Thus, constant balance between the horizontal loads is an advantageous feature for a press machine with a crankshaft having two symmetric eccentric portions.
In the foregoing conventional press machine, however, the stroke length of the slide was always constant and was not adjustable. Depending on an object to be pressed, however, a press machine may require change in a top dead center and a bottom dead center to adjust the stroke length.